Loads on Long Bones

The human skeletal system is a complicated kinematic chain. Loads are transmitted via bones and bone
positions and orientations are controlled by muscles. The largest loads in our body can be found in
the lower extremity of the skeletal system, namely long leg bones. The total load combines body
weight and both external and dynamic loadings.

The investigation and quantification of force distributions in individual muscles and bones during
various activities is complicated. The main reason, to the writer's knowledge, is a deficiency of
non-operative (non-invasive) and remote data acquisition systems that would enable the accurate
measurement of forces inside the human anatomy. Therefore, many investigators have adopted mathematical
modelling and simulation to tackle this problem. The results obtained using such models commonly
overestimate the magnitudes of forces and bending moments [1]. However, these models can be refined
by injecting experimentally-obtained data for muscle and bone properties, and loads. The experimental
data available are very limited. Most experimental data come from measurements on animals and in some
cases on human anatomy post-mortem. The other source of data is orthopaedic fixators, instrumented with
data acquisition systems. Such fixators monitor and measure load patterns and displacements during
the patient's daily activities. However, data obtained using this approach are limited and only indicate
the partial capacity of the limb. Heller et al[2] have measured forces in the
hip and found the peak force is more than 300 % of body weight for both walking and stair climbing. Their
mathematical model was confirmed by in vivo testing and data obtained from hip implants.
Duda et al[3], have modelled load distribution throughout a healthy femur.
Their results indicate that the femur transmits loads of more than 230 % of body weight and bending moments
of up to 20 % of body-weight-metres during walking activity. Schneider et al[4]
have implanted a telemetrized intramedullary nail into the femur with a midshaft fracture in a 33 year old
patient. The maximum measured load in axial bone direction was 120 % of the body-weight and maximum axial
moment of 1.3 % body-weight-metres for a single stance. Anterior-Posterior and Medial-Lateral loads were
of the order of 8 % of body-weight and moments of up to 5 % of body-weight-metres (data derived from graphs
based on body-weight of 750 N). It can be speculated that loads in the tibia are of the same magnitude or
higher, based on the geometry and structure of the bone, and direction of gravity.